1. Field of the Invention
The present invention relates to a brushless DC motor and more particularly, to a variable speed brushless DC motor, which utilizes on/off status of a speed-change switch to control the way of electric current passing through the stator module, so as to further change the torque-rotational speed characteristic curve of the variable speed brushless DC motor, achieving the desired motor rotational speed control.
2. Description of the Related Art
The history of AC motor is over one hundred years, however, the history of brushless DC motor has only several decades. Since mid 1980s, brushless DC motor has been intensively used in instruments. Because brushless DC motor can be made to fit the specifications of different information products, market growth of brushless DC motor keeps going with fast development of information industry. Brushless DC motor still has a great market potential in the near future because of the following two reasons.
(a) World-wide PC market keeps developing, and a new generation of computer with fast CPU (Pentium or Pentium PRO) requires DC fan to dissipate heat.
(b) Many apparatus (car AV equipment, VGA card, etc.) start to use DC fan for cooling, thereby increasing the demand for brushless DC motor.
FIGS. 9 and 10 show a conventional commutation circuit for variable speed single-phase brushless DC motor. This design of commutation circuit is comprised of a power circuit A and a signal circuit B. During voltage supplying of the stator winding power source A1 and on status of the speed-change switch A2, the signal circuit B detects the magnetic pole of the rotor and switches the status of the commutation switches A3, A4, A5, A6, enabling electric current to pass through the stator winding A7, thereby rotating the motor. Further, when the speed-change switch A2 is off, electric current passes from the stator winding power source A1 through the voltage drop power resistor A8 where the voltage is dropped, and then passes to the stator winding A7, thereby lowering the rotational speed of the motor.
This structure of variable speed single-phase brushless DC motor has drawbacks as outlined hereinafter.
1. The use of a voltage drop power resistor A8 in the power circuit A greatly complicates the circuit and increases its size and manufacturing cost.
2. Because the operation of the voltage drop power resistor A8 in the power circuit A releases heat energy, a cooling fan using such a variable speed single-phase brushless DC motor has a low performance.